From U.S. Pat. No. 2,624,245 a method is known for producing continuous webs of paper extensible in a mainly longitudinal direction, i.e. in the direction in which the web extends. This method uses a plant comprising a heated roll having a smooth, preferably chromium-plated surface, and a rubber belt of a certain thickness maintained adhering to the heated roll along a certain portion of its surface by a series of transmission rollers which are at least partly motor-driven. The diameter of the roller which causes the rubber belt to adhere to the commencement of the portion in contact with the heated roll (upstream transmission roller) is small compared with the diameter of said heated roll, such that at their contact generating line the rubber belt undergoes a sudden deviation and then a sudden variation in concavity. The combined effect of this sudden concavity variation, to which the rubber belt is subjected by the upstream transmission roller, which causes said belt to adhere to the heated roll, means that when a paper web under formation, still having a high water content, is introduced between the rubber belt and the heated roll, it is subjected in the portion immediately downstream of the contact generating line to a longitudinal compaction linked to the water quantity contained, to the thickness of the rubber belt, and to the ratio of the diameter of the upstream transmission roller to the diameter of the heated roll.
In its turn, this longitudinal compaction gives the paper web obtained a longitudinal extensibility of about 15% in practice.
However, paper obtained by this method has almost no transverse extensibility, in the sense that the value of this transverse extensibility is of the order of 4-5%, i.e. virtually the same as that found in any traditionally produced paper, for equal mix and refining. It should be also noted that higher values of transverse extensibility can be achieved only by costly refining methods at high density, using complex costly plants with high energy consumption.
From WO2005/100686 a machine is known for producing paper extensible longitudinally and transversely which at the same time is of better smoothness than that obtainable by traditional methods. This known machine comprises a compaction station with a pair of counter-rotating rollers, one of which is of steel, is driven at higher velocity and comprises in its surface a series of circumferentially extending incisions of spiral development, while the other is covered with a rubber layer and is driven at lesser velocity.
When the paper web under formation is made to pass between the two rollers, the combined effect of their different rotation velocity and the presence of the incisions on the steel roller causes longitudinal compaction of the material forming the web and its accumulation within the incisions of the steel roller, and essentially results in the obtaining of a paper web of smooth surface which at the same time possesses good longitudinal and transverse extensibility.
This known solution has proved valid from a theoretical viewpoint but has highlighted limits and drawbacks in its practical implementation, in particular in achieving the high production rate for which current papermaking machines are dimensioned (about 1000 m/min). Taking account of the fact that at the nip between the rubber roller and the steel roller the rubber layer is subjected to local temporary compression, it is clear that a usual rubber roller of diameter 500 mm is subjected to 637 compression cycles per minute, i.e. more than 10 compressions per second. However experimental tests carried out under actual operating conditions have shown that if the number of compressions exceeds 2 per second, the life of the roller suffers, even with the use of complex costly cooling systems.
Hence to reduce to this value the number of compressions of the rubber roller, rollers of a diameter of at least 2500 mm would have to be used, with considerable machine dimension problems and periodic replacement of the rubber layer, an operation which cannot be carried out in the paper works, but only by specialized industries, which are usually not equipped to operate on rollers of such dimensions.
Moreover, rubber-coated rollers of these dimensions present considerable difficulties in their replacement by others with their outer layer of a rubber of different properties such as required for obtaining paper webs of particular characteristics.
From GB 868391 it is also known to produce an extensible paper web by passing the paper web under formation between a rubber roller and a steel roller made to rotate in the opposite directions at the same peripheral velocity. The compression of the paper web on passage between the two rollers results in a temporary local reduction in cross-section, so increasing its passage velocity. When at the exit from the two rollers the paper web under formation regains its original cross-section, the consequent reduction in velocity, accompanied by its adherence to the rubber roller, results in its longitudinal compaction. This is a technical principle totally different from that known from WO2005/100686, which instead uses the different peripheral velocity of two counter-rotating rubber and steel rollers to produce longitudinal compaction of the forming paper web and simultaneous accumulation thereof in incisions provided in the steel roller; and this different principle makes it impossible to combine the teachings deriving from the two aforesaid documents.